Steel production is a very energy intensive industry with much of the energy consumed in hot deformation needed to process the steel into useful shapes. A prime example of such energy use is the production of thin strip where conventional practice requires large ingots to be hot rolled into slabs and then into strip before a final cold roll operation to the desired thickness. In recent years, much of the improvement in efficiency and reduction in energy consumption in the steel industry has resulted from the application of continuous slab casting technology, where steel slabs are produced directly from the melt. While the amount of hot deformation processing is greatly reduced, the need for a hot rolling operation is not eliminated entirely because of the limits on the minimum slab thickness possible using stationary chill molds.
It is recognized that substantial improvement in efficiency can be obtained with near-net-shape continuous casting. One example of near-net-shape continuous casting is thin sheet casting of high quality steel sheet with thicknesses on the order of 0.125 inch. The savings of such a casting technique are expected to be considerable with estimates ranging up to $50/ton. Two examples of techniques for thin sheet casting include planar flow casting and melt overflow casting.
In thin sheet casting techniques, such as planar flow casting and melt overflow casting, a molten metal (such as steel) flows through a nozzle which is submerged into a pool of molten metal. The nozzle geometry directs the flow of the molten metal along a chilled wheel (normally made of copper or a copper alloy) spinning at a relatively high rate of speed (up to 30 ft/sec). The chilled wheel is cooled by circulation of water in channels in the wheel or by the spray of water jets on the interior surface of the wheel. The molten metal on the bottom of the pool is dragged from the pool by the contact with the spinning wheel and solidifies as heat is transferred to the chilled wheel. The metal continues to solidify until the entire sheet is solid and is then removed from the wheel. It is estimated that with respect to steel, thin sheet direct strip casting offers the potential to reduce energy consumption by 17 to 32% over conventional strip fabrication technologies.
Essential to the success of thin sheet casting technique described above is a high rate of heat transfer from the solidifying metal to the chilled wheel. There are at least two reasons why a high rate of thermal transfer might not be maintained. First, centrifugal force acting on the solidifying metal strip can tend to throw the strip off the wheel. The force acting to throw the metal off the wheel is proportional to the metal density, metal thickness, and the square of the wheel speed. Second, there are forces which arise from the metal shrinkage and deformation occurring as the metal solidifies. These forces attempt to peel the solidifying strip away from wheel in small patches leaving gaps between the strip and the wheel. These gaps can seriously degrade the rate of heat transfer.
Of further necessity to melt spinning techniques is the need to carefully control or regulate the flow of metal as it enters upon the wheel. Physical constraints, such as ceramic troughs, channels, or nozzles are of limited use because of their cooling effect on the metal and friction and wear problems.
It is evident that puddle length is the limiting factor for all reasonable casting speeds. Low casting speeds may allow the production of strip thicknesses of up to 0.25 inches. Any techniques which assist in the control of large molten puddle lengths would allow higher casting speeds to be used to produce a given cast thickness or, alternatively, allow a greater cast thickness to be obtained at a given casting speed. Another consideration involves ensuring a smooth upper cast strip surface in order to improve the product quality.
The application of magnetofluidynamic (MFD) technology to the liquid metal pool on the casting surface allows for the exercise of control by the development of a radially oriented hold down body force (electromagnetic pressure) on the molten metal strip. Such a normal force would keep the hot metal in close contact with the water cooled casting substrate and thereby enhance the heat transfer and cooling of the cast product. An electromagnetic pressure applied on a solidifying strip could also be used to enhance the shape and surface quality of a thin metal strip cast upon a chilled casting surface.
Accordingly, it is an object of this invention to use thin strip casting to produce a sheet of metal up to 0.125 inch thick.
It is another object of this invention to utilize an electromagnetic force to hold down metal strip on a chilled casting surface.
It is an object of this invention to employ magnetofluidynamic forces to a solidifying metal strip to enhance the rate of heat transfer and thereby increase the thickness of the strip produced.
It is another object of this invention to ensure that a close thermal contact between a solidifying metal strip and a chill block by means of electromagnetic pressurization.
It is still a further object of this invention to provide a high intensity magnetic field required for flow control.
It is yet another object of this invention to provide an apparatus and techniques which assists in the control of large molten puddle lengths to allow higher casting speeds on a moving chill block to produce a given cast thickness or allow a greater cast thickness to be obtained at a given casting speed.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.